Will I lose weight if I eat like this?

I'm very overweight (like 300 lbs.) and I want to spend this summer losing it. I lose weight pretty quickly if I stick to a set diet plan. This is what I ate yesterday:

Breakfast

~2 egg whites

~2 slices of whole wheat toast

~ 1 light strawberry Activia yogurt (4 oz.)



Snack:

~1 apple



Lunch:

~Tuna sandwich on whole wheat bread



Dinner:

~1 slice of pizza



I know the pizza was a bad idea and I caved a little but believe me, it's better than how much I used to eat when I wasn't on a diet. I'd eat like 3 slices and still want more. And was it too much bread? I only drank diet snapple (I can't stand water). How much exercise should I be doing??? Am I eating too little? I have a tendency to go to extremes when I decide to do something (like lose weight). lol Thanks bunches!!!!! And approximately what was my calorie intake???

If it helps, I'm 15 yrs old. 5' 7"Will I lose weight if I eat like this?
Okay, everything is fine. Just saying, if you eat tuna fish everyday, you'll get mercury poisoning. No, you aren't eating too little, do like half an hour or so of exercise. (I'm not sure how much you're supposed to do) I don't know your calorie intake though...Will I lose weight if I eat like this?
You won't gain wait. Your calorie intake was anything between 950-1250. Try to eat smaller portions and add more fruit and vegetables. Eat a dinner that is rather healthy and contains barley any fat and low calories.

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